Recently, there is an increasing need for display devices such as thin flat panel displays (FPD) instead of display devices with a cathode-ray tube. There are many kinds of FPDs. Known examples of FPDs include non-self-luminous liquid crystal displays (LCD), self-luminous plasma display panels (PDP), inorganic electroluminescence (inorganic EL) displays, and organic electroluminescence (organic EL) displays.
Among them, organic EL displays use thin and light-weighted display elements (organic EL element), and have characteristics such as low voltage driving, high luminance, and being self-luminous. Accordingly, organic EL displays are actively researched and developed.
The organic EL elements are designed to include a pair of electrodes (cathode and anode) on a substrate and an organic layer which is provided between the pair of electrodes and which includes at least a light-emitting layer. The light-emitting layer is obtained by doping a host material with an organic light-emitting material. In general, a hole injection layer etc. obtained by doping a host material with acceptors is provided between the light-emitting layer and the cathode, and an electron injection layer etc. obtained by doping a host material with donor is provided between the light-emitting layer and the anode.
In the organic EL element, a voltage is applied across the cathode and the anode so that electron holes are injected from the cathode into the organic layer and electrons are injected from the anode to the organic layer. The electron holes and the electrons which have been injected from the electrodes recombine in the light-emitting layer to form an exciton. The organic EL element emits light using light radiated when the exciton decays.
In general, the light-emitting layer is made of an organic light-emitting material such as a phosphorescence-emitting material and a fluorescence-emitting material. Since the organic EL element using the phosphorescence-emitting element has advantages such as high light-emitting efficiency and a long life, the organic EL element using the phosphorescence-emitting material for the light-emitting layer has become widely used recently. Furthermore, in order to realize lower power consumption of the organic EL element, there has been developed an organic EL element to which a phosphorescence-emitting material whose internal quantum efficiency is 100% at most is introduced.
The organic EL element that emits red light and the organic EL element that emits green light employ phosphorescence-emitting materials whose internal quantum efficiency is 100% at most. However, the organic EL element that emits blue light does not employ a phosphorescence-emitting material whose internal quantum efficiency is 100% at most, and instead employs a phosphorescence-emitting material whose internal quantum efficiency is 25% at most.
The organic EL element requires higher energy in blue emission than in red emission and green emission. Furthermore, in order to obtain the energy from excited triplet level (T1), it is necessary to confine all of T1, electrons, and electron holes into the phosphorescence-emitting material in the light-emitting layer. Accordingly, not only in the material constituting the light-emitting layer but also in the material surrounding the light-emitting layer, the gap between Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) is required to be very large. However, since the gap between HOMO and LUMO of the light-emitting layer is required to be large, the host material for the light-emitting layer is difficult to be a material which has molecular conjugation, molecular interaction, and high mobility of carriers. Consequently, when a blue phosphorescence-emitting material is used, a high voltage is required for driving but light-emitting efficiency is low for the high voltage.
FIG. 8 shows a specific example of a conventional organic EL element 31 employing a blue phosphorescence-emitting material. FIG. 8 shows an energy diagram of individual layers constituting the conventional organic EL element 31 employing a blue phosphorescence-emitting material. In the drawing, the host material for an electron hole injection layer 33 is NPB (HOMO level=5.5 eV, LUMO level=2.4 eV), the host material for an electron hole transportation layer 34 is mCP (HOMO level=5.9 eV, LUMO level=2.4 eV), and the host material for an electron transportation layer 36 is 3TPYMB (HOMO level=6.8 eV, LUMO level=3.3 eV). The phosphorescence-emitting material for the light-emitting layer 35 is FIr6 (HOMO level=6.1 eV, LUMO level=3.1 eV). In order to confine electron holes and electrons to the FIr6, the host material for the light-emitting layer 35 is UGH2 having a wide gap between HOMO level and LUMO level (HOMO level=7.2 eV, LUMO level=2.8 eV). However, since UGH2 has a wide gap, it is impossible to efficiently transport electron holes from the electron hole transportation layer 34 to the light-emitting layer 35. Similarly, it is impossible to efficiently transport electrons from the electron transportation layer 36 to the light-emitting layer 35. Consequently, as described above, the organic EL element 31 employing a blue phosphorescence-emitting material requires a high voltage for driving the organic EL element 31 but exhibits low light-emitting efficiency for the high voltage.
As such, in order to improve light-emitting efficiency of the organic EL element employing a blue phosphorescence-emitting material, various ingenuities have been proposed. For example, Non-patent Literature 1 discloses an organic EL element including two light-emitting layers. Specifically, the organic EL element includes an organic layer consisting of an electron hole injection layer, a first light-emitting layer, a second light-emitting layer, and an electron injection layer in this order between a pair of electrodes. In Non-patent Literature 1, the host material for the electron hole injection layer is DTASi (HOMO level=5.6 eV, LUMO level=2.2 eV), and the host material for the electron injection layer is Bphen (HOMO level=6.4 eV, LUMO level=3.0 eV). Furthermore, the host material for the first light-emitting layer is 4CzPBP (HOMO level=6.0 eV, LUMO level=2.5 eV), and the host material for the second light-emitting layer is PPT (HOMO level=6.6 eV, LUMO level=2.9 eV). The first light-emitting layer and the second light-emitting layer are doped with FIrpic (HOMO level=5.8 eV, LUMO level=2.9 eV) which is a blue phosphorescence-emitting material.
This configuration provides an organic EL element whose gap between HOMO level and LUMO level in the first light-emitting layer and the second light-emitting layer is small. Accordingly, the light-emitting layer may be made of a host material that improves mobility of electron holes and electrons. This is because transportation of electron holes and electrons in an organic deposition film is made in the form of hopping conduction (Non-patent Literature 2). In order that transportation of electrons between molecules is made in the form of hopping conduction, wave functions of two electronic states, neutral state and radical anionic state, are required to overlap significantly. On the other hand, in order that transportation of electron holes between molecules is made in the form of hopping conduction, wave functions of two electronic states, neutral state and radical cationic state, are required to overlap significantly. That is, as stacking between the neutral state and the radical anionic state or stacking between the neutral state and the radical cationic state (π-π interaction) is more significant, mobility of electron holes and electrons is higher. Furthermore, as the stacking is more significant, the gap between HOMO and LUMO is smaller. Accordingly, this configuration provides an organic EL element which can be driven by a low voltage of 4.6 V and which exhibits high light-emitting efficiency of 22 cd/A at 1000 cd/m2.